The disclosed invention relates in general to frequency discriminators suitable for use in measuring phase noise in tunable ocsillators and more particularly to frequency discriminators utilizing delay lines. Measuring and specifying phase noise has become increasingly important as phase noise is the limiting factor in many RF and microwave systems like Doppler radar and space telemetry systems and communication links. Several different types of frequency discriminators have been developed including lumped resonators for the UHF band, transmission lines for the VHF, UHF and microwave bands and waveguide stubs and cavity resonators for the high microwave band. Of these schemes, the transmission line discriminators exhibit the best dynamic range for practical phase noise measurements. Such discriminators are disclosed in U.S. Pat. Nos. 4,002,969 issued to Barley et al on Jan. 11, 1977, 4,002,970 issued to Ashley et al on Jan. 11, 1977 and 4,002,971 issued to Rast et al on Jan. 11, 1977.
In these transmission line discriminators, a signal under test is divided by a power divider into a signal channel component and a power channel component, each of which is applied to an input of a mixer. A transmission line is included in the signal channel between the power divider and the mixer to convert frequency fluctuations into phase fluctuations. The output signal from the mixer has components at the sum and at the difference of the frequencies of the signal under test. The components at the sum of the frequencies are discarded by a filter or by the inherent bandwidth of the circuit elements that are responsive to the output of the mixer so that only the difference signal (referred to herein as the intermediate frequency or IF component) is used to observe the phase noise. For a single frequency carrier signal under test having some frequency noise (converted to phase noise by the delay line), the resulting output signal has a DC component proportional to the square of the carrier signal, an AC component proportional to the phase noise and some negligible AC components proportional to higher powers of the phase noise. The DC component is eliminated by assuring that the signal channel component and the power channel component are in quadrature (i.e., are ninety degrees out of phase plus additional phase variations due to the phase noise). The proportionality constant (known as the discriminator constant) between the AC component of the mixer output signal and the frequency noise signal is determined by a calibration of the discriminator.
These transmission line discriminators, when used for phase noise measurements in the VHF and UHF bands, require a variable phase shifter in either the signal channel or the reference channel to obtain quadrature between the reference path and the delayed signal path. This phase shifter adds both expense and noise to the discriminator. In addition, the calibration procedure to determine the discriminator constant is lengthy and usually involves controlled known frequency modulation of the signal under test, thereby requiring extra equipment including an extra signal source. For maximum discriminator sensitivity, there exists an optimum transmission line length which gives a total loss of 8.7 dB. At 500 MHz, this is equivalent to about 70 meters of excellent quality coaxial cable allowing about 300 nanoseconds of delay. Because loss increases with frequency, transmission line discriminators cannot be used for measuring phase noise on carrier frequencies above 5 GHz. Furthermore, because transmission line discriminators have to be calibrated at different frequencies, they are impractical to use for phase noise measurements of tunable oscillators.